a) Field of the Invention
This invention relates to optical cable.
b) Prior Art
Conventional optical cables have optical fibers extending along passages within which, generally, the fibers have freedom for lateral movement. In such cables there is a requirement for steps to be taken to avoid damage caused by water penetration. Fiber subjected to contact by water is known to experience a degradation in strength which, because of improvements in fiber coating techniques, is not of serious practical concern. However, there is a problem with customer perception of this type of situation in that it is believed that water degradation of fibers is a reason for concern. Further, there is the possibility that water inside a cable will freeze. The expanding ice will then compress individual fibers resulting in signal attenuation and fiber breakage.
In order to prevent water penetration along cables, it is known to use a water blocking grease or gel which surrounds the optical fibers. Unfortunately, this water blocking material is an irritant to water blocking installers who need to clean the grease or gel from the fibers before they may be handled or spliced. Grease or gel make it difficult and unpleasant to handle the fibers during installation or repair of a cable and low temperature (e.g. below 0.degree. C.) change in viscosity of the grease or gel surrounding and contacting fibers may increase signal attenuation in the fibers. One method of eliminating the need for grease or gel in contact with optical fibers while still providing a water blocking function involves the use of a replacement water blocking material which is swellable upon contact with water so as to fill and water block any passageway within which it is disposed. A commonly referred to material having this function is polyacrylate which is normally used in particulate form. In one disclosure, U.S. Pat. No. 5,157,752 a swellable water blocking means is used which comprises an elongate element which swells upon contact with water. This elongate element is disposed within a passageway of the cable together with the optical fibers so that upon contact with water the elongate element swells to block the passageway. In the examples given in this previous patent, the elongate element comprises a carrier which may be of filamentary structure, e.g. a string, or a tape which may be of open construction and the string or tape actually carries the particulate polyacrylate material. Alternatively, no particulate material is used and the elongate member comprises polyacrylate filaments spun with other filaments e.g. polyester or nylon to form a string.
The problem which is found with water swellable materials as the use of a water block is that such materials allow water to remain in the presence of the optical fibers and the degradation problems referred to above may still be found with this type of arrangement. It is also of concern that this standing water may freeze thus resulting in the breakage and signal attenuation problems referred to above. It follows from this therefore that the use of grease or gel as a water blocking material may still be preferred over water swellable water blocking materials even though the disadvantage of needing to clean the fiber of the grease or gel still remains.